Pressure transducers with hydraulic pressure transmission include usually a hydraulic path extending between a process diaphragm, or membrane, and a pressure measuring cell, with the process diaphragm being exposed to a process medium, whose pressure is to be ascertained. Problematic are, in such case, situations, in which high temperatures of the process medium occur at a low process pressure, for example temperatures over 200° C. at a pressure below 100 mbar. Under these conditions, the transmission liquid in the hydraulic path can evaporate, or outgas, as the case may be. This can, in the most favorable case, occur reversibly according to the vapor pressure curve for the particular transmission liquid, with, however, in such case, plastic deformation of the process diaphragm being possible, which can lead to an erroneous measurement. Frequently, the transfer liquid behaves, however, not according to the vapor pressure curve for the pure state, for, due to reactions with impurities or with the surfaces bounding the hydraulic path, the transmission liquid can contain volatile decomposition products, which, following outgassing, no longer go back into solution.
As a result, it is advantageous to utilize transmission liquids which are as temperature stable as possible and have a sufficiently low vapor pressure. Additionally, the transmission liquid should be charged into the hydraulic path in such a way that it remains chemically stable. This is, however, often opposed by other boundary conditions. For example, electronic components and a semiconductor measuring cell cannot be subjected to the high temperatures advantageous for charging. For the charging temperature should, in the ideal case, not lie beneath the maximum process temperature, and hydraulic transmission systems are utilized precisely because of the desire to protect the electronic components and measuring cell from the high process temperatures.